Method for determining NOx mass flow from characteristics map data with a variable air inlet and engine temperature

ABSTRACT

A method for determining the NOx mass flow at the input of an NOx storage catalytic converter in the exhaust gas of an internal combustion engine from operating parameters of the internal combustion engine is introduced, characterized in that the intake air temperature and the engine oil temperature are considered in the determination.

This application is the national stage of PCT/DE01/03250, filed Aug. 24,2001, designating the United States.

BACKGROUND OF THE INVENTION

A method for engine control for gasoline-direct injection internalcombustion engines having NOx-storage catalytic converters is alreadyknown from U.S. Pat. No. 6,119,449. The method provides for a modelingof the NOx-storage catalytic converter and a control (open loop and/orclosed loop) of the change between storage operation and regenerationoperation as well as a catalytic converter diagnosis. An essentialelement is the computation of the NOx raw mass flow at the input of thecatalytic converter from characteristic field data via the inputquantities engine rpm, relative fuel mass entry (referred to full load),exhaust-gas recirculation component and desired lambda.

The differences in the intake air temperature, for example, −20° C. in aScandinavian winter and +40° C. in the tropical or subtropical summerand in the engine block temperature are not considered in the knowncomputation. The engine block temperature corresponds, for example,during a cold start to the ambient temperature and can increase at fullload to the regions of the maximum permissible engine oil temperature.

In a known control for gasoline-direct injection engines, a temperature(Tein) is determined from the intake air temperature and the enginetemperature. This temperature is characteristic for the enclosed gasmixture at the start of the compression.

SUMMARY OF THE INVENTION

With this background, it is the task of the invention to improve themodeling of the NOx raw mass flow at the input of the catalyticconverter.

This task is solved by considering the intake air temperature and theengine oil temperature when modeling.

The intake air temperature and the engine oil temperature are availablein modern engine controls as measurement signals.

According to the invention, the NOx mass flow at the catalytic converterinput is computed with greater accuracy than previously with the aid ofthese data and with an NOx emission characteristic field as a functionof engine rpm, the relative fuel mass, the exhaust-gas recirculationrate and the desired lambda value. A further increase of the accuracy ismade possible in an advantageous embodiment by considering the watervapor content of the intake air.

One embodiment of the invention provides that the NOx mass flow (msnovk)forward of the catalytic converter is computed as the product of a basevalue (msnovk0) and a temperature-dependent factor(exp(FNOXBAKT*delTemv). The base value (msnovk0) is referred to as adefined normal state.

A further embodiment provides that the temperature-dependent factor isproportional to a change (delTemv) of the combustion temperature as aconsequence of changes of an inlet temperature quantity (Tein) which iscombined from air temperature and engine temperature.

A further embodiment provides that the computation of the NOx mass flowahead of the catalytic converter in an internal combustion engine takesplace in stratified operation differently than in homogeneous operation.The internal combustion engine can be operated in a first operating modewith a layered mixture distribution in the combustion chamber(stratified operation) and in a second operating mode with a homogeneousmixture distribution in the combustion chamber (homogeneous operation).

According to a further embodiment, a factor for a lambda-dependent NOxformation activation is considered in the homogeneous operation.

A further embodiment provides that an increasing water vapor componentof the intake air acts in the computation to reduce the NOx mass flowforward of the catalytic converter.

The invention is also directed to an electronic control arrangementwhich executes the above-mentioned methods.

The formulation of the present invention is so simply structured that itcan be integrated into the engine control without great difficulty andwithout it being necessary to implement an existing NOx emissioncharacteristic field additionally for deviating intake air and enginetemperatures. Stated otherwise, a base characteristic field, which isreferred to a normal state and/or exhaust-gas test conditions, can beused further. Deviations of the actual state from the normal state areconsidered by logically coupling the characteristic field values tocorrective values, especially via a multiplicative logic coupling.

In this way, the computation of the NOx mass flow can supply the desiredadditional information more rapidly and with less use of storage space.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein.

FIG. 1 shows the technical background of the invention.

FIGS. 2 to 4 show embodiments of the invention in the context offunction blocks.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, 1 represents the combustion chamber of a cylinder of aninternal combustion engine. The inflow of air to the combustion chamberis controlled via an inlet valve 2. The air is drawn in by suction viaan intake manifold 3. The inducted air quantity can be varied via thethrottle flap 4 which is driven by a control apparatus 5. The followingare supplied to the control apparatus: signals as to the torque commandof the driver, for example, as to the position of an accelerator pedal6; a signal as to the engine rpm (n) from an rpm transducer 7; a signalas to the quantity ml of the inducted air by an air quantity sensor 8;and, a signal US as to the exhaust-gas composition and/or theexhaust-gas temperature from an exhaust-gas sensor 12. Exhaust-gassensor 12 can, for example, be a lambda probe whose Nernst voltageindicates the oxygen content in the exhaust gas and whose internalresistance is applied as an index for the probe temperature, exhaust-gastemperature and/or catalytic converter temperature. The exhaust gas isconducted through at least one catalytic converter 15 wherein toxicsubstances are converted from the exhaust gas and/or temporarily stored.

The control apparatus 5 forms output signals from the above and, ifneeded, additional input signals as to additional parameters of theinternal combustion engine, such as input air temperature, coolanttemperature, et cetera. These output signals are for the adjustment ofthe throttle flap angle α via an actuating member 9 and for driving afuel injection valve 10 via which fuel is metered into the combustionchamber of the engine. Furthermore, the triggering of the ignition viaan ignition device 11 is controlled by the control apparatus.

The throttle flap angle α and the injection pulse width ti are essentialactuating quantities, which are to be matched to each other, forrealizing the wanted torque, the exhaust gas composition and theexhaust-gas temperature. A further essential actuating variable forinfluencing these quantities is the angular position of the ignitionrelative to the piston movement. The determination of the actuatingquantities for adjusting the torque is the subject matter of U.S. Pat.No. 6,512,983 which is to this extent incorporated into the disclosure.

Furthermore, the control apparatus controls additional functions forachieving an efficient combustion of the air/fuel mixture in thecombustion chamber, for example, an exhaust-gas recirculation and/ortank venting (not shown). The gas force, which results from thecombustion, is converted into a torque by piston 13 and drive 14.

For modeling the NOx entry into the catalytic converter, the controlapparatus computes the mass flow mNox of the nitrogen oxides to thecatalytic converter for oxygen excess in the exhaust gas from operatingparameters of the combustion engine. This can, for example, take placeby addressing a characteristic field wherein the instantaneous NOxemissions are stored in dependence upon the operating point. Essentialoperating parameters in this context are the engine rpm, the relativefuel mass, the exhaust-gas recirculation rate and the desired lambdavalue. The total quantity of nitrogen oxide, which is emitted in thelean phase, results therefrom via integration.

Details of the modeling of the NOx mass flow, which results from thecombustion, are described in U.S. Pat. No. 6,119,449 which isincorporated to this extent into the disclosure.

The additional influence quantities, which are used in the context ofthe present invention, are intake air temperature Tans and engine oiltemperature Tmotor. These influence quantities act on the combustiontemperature of the air/fuel mixture which decides as to the NOxformation in lean operation. Under these conditions, NO is formed inaccordance with the Zeldovich mechanism (thermal NO). With the knownactivation energies of this mechanism, one can estimate that, forotherwise same conditions, the temperature-dependent exponential term ofan Arrhenius equation can be linearized for usual inlet temperatures(equation 1). For considering intense effects such as, for example, athigh air humidity, the use of a characteristic line on the basis of thegiven exponential function is recommended. With this starting point, onecan adapt the influence factor FNOXBAKT for the effect of the differenttemperatures on the NOx mass flow to each type of vehicle. In this way,the formulation is also independent of the NOx formation mechanism andapplies, for example, also to the prompt-NO, which forms (preferablyenriched), for example, with significantly less activation energy thanthe Zeldovich-NO.

For computing the NOx mass flow forward of the catalytic converter(msnovk), the following formulas are suggested (equations 1-2):

msnovk=msnovk0*exp(FNOXBAKT*delTemv)(linearized:msnovk=msnovk0*(1+FNOXBAKT*delTemv)).  (1)

Here, msnovk0 corresponds to the NOx mass flow forward of the catalyticconverter from the above-mentioned characteristic field and FNOXBAKTcorresponds to a factor which represents the temperature-dependent NOxformation activation. For example: FNOXBAKT=0.003/Kelvin.

delTemv=FT0_TV * (Tein−TBEZUG)−FTWASDA * antwasda.  (2)

delTemv corresponds to a change of the combustion temperature because ofinlet effects. Tein represents an inlet temperature quantity, combinedfrom air temperature and engine temperature. TBEZUG corresponds to atemperature Tein at which the characteristic field was recorded. In theevent that TBEZUG for the characteristic field is not constant, theformation of a mean value and, if needed, a recomputation of individualcharacteristic field points to this mean value takes place in advance offeeding the characteristic field into the engine control. FT0_TVrepresents an amplification factor for the effect of Tein changes on thecombustion temperature. Example: FT0_TV=2.1. The quantity antwasdaindicates the water vapor component of the intake air in volume percent(absolute). Here, it can be a measurement quantity. The factor FTWASDAindicates the influence of the vapor component on the combustiontemperature. Suggestion: FTWASDA=26.6. The quantity Tans is the measuredtemperature of the intake air of the internal combustion engine andTmotor is the measured engine oil temperature.

The numerical data are based on coarse estimates. These data are, fromcase to case, to be adapted to the conditions of a specific vehicle.

In stratified operation, one proceeds from the situation that theprimary component of the NOx is formed in a local lambda range about thestoichiometric point (lambda =1) because here, the highest temperaturesoccur. One such region always occurs for stratified charge independentlyof the mean lambda. With the mean lambda, only its spatial expansionchanges the effect of what is already considered in the characteristicfield while the characteristic of the temperature dependency of the NOxformation remains essentially unchanged because of the allocation to thelocal region with lambda in the vicinity of 1. For this reason, FNOXBAKTshould remain constant as a parameter in accordance with the input ofdata to the engine control for a specific engine in stratified operation(equation 1).

In homogeneous combustion, the local peak temperature is greatlydependent upon the air/fuel ratio which is characterized by the airnumber lambda. For this case, equation 1 is modified because of thetemperature dependency of the activation factor (equation 1a).

msnovk =msnovk0*exp(FNOXBAKTL*lambda**2* delTemv).  (1a)

Here, FNOXBAKTL defines a factor for a lambda-dependent NOx formationactivation. For example: FNOXBAKTL =0.003/K.

These formulas proceed from the characteristic field NOx mass flowmsnovk0 and use the measurement quantities: intake air temperature Tans,engine oil temperature Tmotor and water vapor component antwasda for themodification of the read-out characteristic field values. The advantagesof the available characteristic field are retained (equation 1) and aresupplemented by a simply structured description of the influence of thementioned measurement quantities. Characteristic for the method is thebundling of all measurement quantities in the ancillary quantity delTemvwhich serves as an index for the increase of the combustion peaktemperature (equation 2). The water vapor component antwasda of theintake air acts as a heat sink and lowers, for a constant air ratio, thecombustion temperature compared to dry air. For the case wherein onedoes not measure the water vapor component, the following considerationcan make the explicit inclusion thereof unnecessary: the maximumabsolute air moisture is strongly coupled to the air temperature in theintake air temperature field from −30° C. to +50° C. By selecting a meanrelative humidity, it is achieved to reduce the problem to theadaptation of the remaining parameters. The deviation of a computedquantity “inlet temperature Tein ” from the temperature TBEZUG isamplified by the factor FT0_TV (equation 2) in the course of compressionand combustion. The effects of the intake temperature Tans and theengine temperature Tmotor are combined in the inlet temperature Tein andthe characteristic field was recorded at the temperature TBEZUG.

Referring to FIG. 2, and as an alternative to the formulas (equations 1and 2), the use of a characteristic field as to the input temperatureTein and the water vapor content of the intake air is considered for thefactor for the multiplication by the NOx characteristic field data orthe multiplication by two characteristic lines. One characteristic lineis for Tein and a second characteristic line is for the water vaporcontent in the intake air. Block 2.1 represents a characteristic fieldwhich is addressed via engine operating variables such as load and rpmand which supplies a first value for the NOx mass flow ahead of thecatalytic converter. The blocks 2.2 and 2.3 represent characteristiclines for corrections of the temperature influence (formed while usingvalues for the engine oil temperature and intake air temperature) and ofthe influence of the water vapor content in the intake air (absolutehumidity) which are logically coupled to each other in block 2.4 andlogically coupled in a block 2.5 with the output value of thecharacteristic field 2.1.

In FIG. 3, for the case that the water vapor content is not measured,one can use a characteristic line as to Tans (in the determination ofwhich the relative humidity was assumed at a mean value of, for example,0.5) and can consider the influence of the engine temperature (FIG. 3)with a simply structured factor (equation 3) for whose derivation thelinearization capability of equation 1 is used.

msnovk=msnovk0*Kennlinie(Tans) * (1+FTMOT*(Tmotor-Tans))   (3)

FIG. 3 shows a function block diagram of this computation. The use of acharacteristic line as a function of Tein is not recommended herebecause the engine temperature has no influence on the air humidityeffect. The factor FTMOT contains the proportional influence of theengine temperature on Tein, its amplification in the compression phaseand a factor for the NOx formation activation FNOXBAKT (suggestion:FTMOT=0.001/K). FIG. 4 provides a summary of the influence quantities ofFIG. 3 in a characteristic field.

An expansion of the present NOx emission characteristic field into twoor three further dimensions is conceivable; however, this is notconsidered because of storage space problems and computation timeproblems.

What is claimed is:
 1. A method for determining the NOx mass flow(msnovK) in the exhaust gas of an internal combustion engine at theinput of a storage catalytic converter from operating parameters whichinclude the intake air temperature and the engine temperature, themethod comprising the steps of: determining the water vapor component ofthe intake air from the temperature of the intake air, wherein saidwater vapor component is factored into the determination of the NOx massflow (msnovK) in the exhaust gas of an internal combustion engine. 2.The method of claim 1, wherein the NOx mass flow (msnovk) forward of thecatalytic converter is computed as a product of atemperature-independent base value (msnovk0) and a temperature-dependentfactor (exp(FNOXBAKT*delTemv)).
 3. The method of claim 2, wherein thetemperature-dependent factor is proportional to a change (delTemv) ofthe combustion temperature as a consequence of changes of an inlettemperature quantity (Tein); and, the inlet temperature quantity (Tein)is combined from air temperature and engine temperature.
 4. The methodof claim 1, wherein the computation of the NOx mass flow forward of thecatalytic converter takes place in stratified operation differently thanin homogeneous operation for an internal combustion engine which can beoperated in a first operating mode with a layered mixture distributionin the combustion chamber (stratified operation) and in a secondoperating mode with a homogeneous mixture distribution in the combustionchamber (homogeneous operation).
 5. The method of claim 4, wherein, inhomogeneous operation, additionally a factor is considered for alambda-dependent NOx formation activation.
 6. The method of claim 1,wherein an increasing water vapor component of the intake air operatesin the computation to effect a reduction of the NOx mass flow forward ofthe catalytic converter.
 7. An arrangement for determining the NOx massflow (msnovK) in the exhaust gas of an internal combustion engine at theinput of a storage catalytic converter from operating parameters whichinclude the intake air temperature and the engine temperature, thearrangement comprising: means for determining the water vapor componentof the intake air from the temperature of the intake air; and, means forfactoring the water vapor component of the intake air into thedetermination of the NOx mass flow (msnovK) in the exhaust gas of aninternal combustion engine.